1. Field of the Invention
The present invention relates to an image transmission system, and more particularly to a method for varying a quantization parameter (QP) according to significance of an object by using an object-based bit rate control system.
2. Background of the Related Art
In general, when a transmission bandwidth is restricted or varied in an image transmission system, a bit rate is controlled according to conditions of a given channel to efficiently transmit an image data including plenty of information through the channel. Exemplary bit rate control methods include a forward control method for allocating a bit rate in consideration of characteristics of an input image, and a backward control method for determining a parameter of an encoder in consideration of characteristics of a back terminal of a source encoder (namely, output terminal) such as condition information of a buffer (for example, bit rate).
An image encoder for low-speed transmission media such as image communication or videophone communication must show superior performance in compression efficiency and low complexity. Therefore, most of the image encoders for low-speed transmission media employ the backward control method for controlling the bit rate by regulating the quantization parameter (QP) considering conditions of the buffer.
FIG. 1 is a schematic structure view illustrating a conventional bit rate control system. The system includes a characteristic analyzer 101 for analyzing characteristics of an input image, a quantizer 102 for encoding the image, an output buffer 103 for outputting the encoded image as bit rows, a target bit allocator 104 for allocating a target bit on the basis of condition information of the characteristic analyzer 101 and the output buffer 103, and a bit rate regulator 105 for regulating a bit rate according to the allocated target bit.
The characteristic analyzer 101 analyzes the characteristics of the image data and provides the result to the target bit allocator 104, and the target bit allocator 104 allocates the target bit according to the characteristic result of the image data and the condition information of the output buffer 103. The bit rate regulator 105 regulates the bit rate by using the QP of the quantizer 102 according to the target bit allocated by the target bit allocator 104, and the quantizer 102 quantizes the image data according to the regulated bit rate and stores the encoded data in the output buffer 103. The output buffer 103 adjusts the bit rows to be outputted at a predetermined speed and performs functions for preventing overflow or deficiency of a buffer, which may generate during bit row reconstitution.
Here, the bit rate regulator 105 uses the QP as an encoding parameter for regulating the bit rate. For example, when the QP has a high value, the bit rate is lowered but the quality of image is reduced. In the case that the QP has a low value, the bit rate is increased but the quality of image is improved. That is, the quality of image and the bit rate are contrary to each other. Accordingly, when a bandwidth of the network is reduced to lower the bit rate, the quality of image is reduced.
However, the conventional bit rate control method usually suggests one arithmetic model (for example, linear model, non-linear model, Laplician mode, exponential mode, Gaussian mode, etc.) for the whole images, and allocates the bit according to the model. It is thus impossible to decide an optimal QP for the characteristics or significant region (for example, face, eyes, nose or mouth of a person) of the input image. That is, even if the region is significant, when the bit rate is lowered due to poor network environment, the quality of image is reduced. In addition, even if the region is not significant, it can have higher image quality than the significant region due to variations of the network environment.
The correlation between the bit rate and the image quality must be considered in the image communication or videophone communication. That is, the user is more sensitive to reduction of image quality of the significant or interest region than the insignificant or non-interest region. The image quality of the significant or interest region is a more important factor by which the user judges the quality of image than the image quality of the full screen.
The conventional bit rate control method uniformly encodes the full screen according to the QP, and thus provides the user with poor quality of image in the significant region which the user is sensitive to.
Moreover, the conventional bit rate control method regulates the QP according to overflow or deficiency of the output buffer, and thus fails to provide the user with the high quality of image due to big differences and reduction of the quality of image according to conditions of the network